The present invention relates to the application of catalyst or catalyst support coatings to porous ceramic catalyst supports of the kind used for the control of exhaust emissions from internal combustion engines. More particularly, the invention relates to pre-coating or passivation procedures that enable the application of catalyst coatings to such ceramic catalyst supports while avoiding the harmful effects of those coatings on the physical properties of the ceramic supports.
The adverse effects of catalyst coating processes on the thermal properties of ceramic catalyst supports such as ceramic honeycombs are well known, and numerous solutions to those problems have been suggested. The principal problem is that oxide constituents of the catalyst or catalyst support coatings, such as alumina, will penetrate into the microstructure of the ceramic supports during the coating and curing processes, filling that microstructure in a manner that typically increases the thermal expansion coefficient of the catalyzed honeycombs. These thermal expansion increases substantially reduce the thermal shock resistance of the honeycombs. U.S. Pat. Nos. 4,452,517, 4,483,940, 4,532,228 and 5,346,722 describe this problem and various solutions thereto.
Among the honeycomb structures presently used for internal combustion engine emissions control, specifically to provide for the efficient removal of particulates such as soot from the exhaust stream, is a type of structure referred to in the art as a wall-flow filter. Such filters are typically porous ceramic honeycombs with end-plugs provided in alternate channels, whereby exhaust gases traversing the structures must pass through the porous channel walls to capture the particulates prior to exhaust discharge. Examples of ceramic materials useful for making such filters, but which can be adversely affected by catalyst coating processes, are cordierite, aluminum titanate, silicon carbide, refractory alkali zirconium phosphates, and low-expansion alkali aluminosilicates such as beta-eucryptite, beta-spodumene, and pollucite. Examples of exhaust filter designs employing these materials are disclosed in U.S. Pat. Nos. 6,620,751, 6,673,414 and 6,468,325.
To address tightening diesel engine emissions regulations being adopted in the United States and Europe, recent attention has focused on basic improvements in the design and performance of ceramic wall-flow honeycomb filters for treating diesel engine exhaust gases. Among other improvements, design changes allowing for the use of catalyst coatings to control hydrocarbon and/or nitrogen oxide emissions are being implemented. The goal is to develop an improved high-temperature-resistant, high-thermal-shock-resistant, low cost honeycomb soot filter compatible with advanced emissions control catalyst technologies that can replace current high-cost and/or uncatalyzed particulate filters.
Materials and methods successfully used in the prior art to minimize the adverse effects of catalyst coating processes on the thermal expansion coefficients and thermal shock resistances of flow-through ceramic honeycomb catalyst supports have proven largely unsuitable for the production of catalyzed wall-flow filters. A persistent problem is the need to maintain high gas permeability as well as a low coefficient of thermal expansion in the catalyzed filters, while still providing a catalyst loading sufficient for effective catalytic treatment of the exhaust stream. The catalyst coatings must be disposed within the filter structure in such a way that they provide an effective distribution of catalyst without unacceptably degrading either the thermal expansion coefficients or the required high gas permeabilities of the supporting ceramic wall structure.
Desirably, increases in CTE resulting from the application of washcoats or catalyst coatings should not exceed 10×10−7/° C. as measured at a temperature of 1000° C., and CTE values for the washcoated filters should not exceed 25×107/° C. as measured at that temperature, in order to preserve the thermal shock resistance of the filter. Further, gas permeabilities through the catalyzed filter should be sufficient to maintain pressure drops below 8 kPa at exhaust gas space velocities up to 150,000 hr−1 after filter regeneration to remove trapped particulates.